Journal of Astronomical History and Heritage, 10(1), 31- 48 (2007). E.E. BARNARD AND THE ECLIPSE OF IAPETUS IN 1889
James Bryan McDonald Observatory, 1 University Station, C1402, Austin, Texas 78712-0259, USA E-mail: [email protected]
Abstract: On 1-2 November 1889 E.E. Barnard observed Iapetus as it passed through the shadows of Saturn and its ring system. Over 2.6 hours he produced 75 differential visual magnitude estimates of Iapetus relative to Tethys and Enceladus. The resulting light curve demonstrated the C ring's already known translucence, but it also showed something unexpected. As Iapetus crossed the sunlit gap between Saturn’s upper atmosphere and the C ring’s inner boundary, instead of remaining constant in brightness, the satellite steadily faded. Apparently, it passed through a shadow, but in 1889 nothing was known to exist in this space. Barnard dismissed the effect as unreal. Although he could not have known, his light curve also implied greater density in the C ring than exists today near the B-C ring boundary. What is the significance of his observation? Were Barnard’s visual magnitude estimates wrong? Was the inner ring system significantly different in 1889? Did Barnard observe an event that temporarily affected the ring’s density in the line of sight? There are no conclusive answers because he observed the eclipse alone and visually. Yet his method of observation and light curve are thought-provoking. What he recorded con- forms in certain ways to the presence of spokes on Saturn’s rings. Spacecraft have observed spokes only on the B ring, but visual observers as early as 1873 have seen spokes and spoke-like objects in the A, B and C rings. I speculate on the possibility that Barnard observed spoke shadows intermingled with ring shadows on Iapetus in eclipse.
Keywords: Barnard, Saturn, Iapetus, spokes, Lick Observatory
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31 James Bryan E.E. Barnard and the 1889 Eclipse of Iapetus
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Figure 3: James Clerk Maxwell (1831–1879) as photographed in 1855 at age 24 while a Fellow of Trinity College Cambridge. In his Adams Prize Essay for 1856 he demonstrated mathematically that Saturn's rings would remain stable as a system of particles. However, when it came to the collisional environment of a vast number of particles with unknown sizes and shapes, Clerk Maxwell had no mathematics to explain particle motions. He privately speculated that the equatorial region of Saturn might be constantly bombarded by particles that were displaced from their orbits by collisions. By 1889 many, including Barnard, had accepted Clerk Maxwell's hypothesis of particulate rings, but they apparently did not know his equatorial bombardment hypothesis. Barnard disbelieved and ignored his own evidence for activity between the C ring and the planet that he obtained during the eclipse of Iapetus. If he had known Clerk Maxwell's concept of equatorial bombardment, would this have influenced his interpretation? Figure 2: Edward E. Barnard (1857–1923) as photographed in (Courtesy: Master and Fellows of Trinity College, Cambridge). 1885 at age 28. The eclipse of Iapetus was one of several unique observations that Barnard made at Lick Observatory from 1888 to 1895. These secured his reputation as a pre- ."$&90$;!*&$9!;4$,4$&90$"3"0&00"&9$70"&'(2$3"&0(0*&$3"$ eminent visual observer (courtesy: Mary Lea Shane Archives %!&'("$ :!*$ 93@9<$ $ R5*0(10(*$ *!:$ &:,$ ,513,'*$ 5(3@9&$ of the Lick Observatory, University of California, Santa Cruz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`$=?aD$ 93*$!//(,!79$&,$%$I"#$:!*$",&$!*$(3@,(,'*$!*$3&$7,';#$ BC3@'(0$\D$/(,13#0#$&90$,";2$0"&(2$&,$&90$7,+/0&3&3,"$3"$ 9!10$ 500"-$ !&$ Q37M$ 90$ 9!#$ !"$ ,//,(&'"3&2$ &,$ 3+/(,10$ &9!&$20!(<$$."$93*$4!+,'*$*,;'&3,"$4,($9,:$%!&'(")*$(3"@$ :90"$90$,5*0(10#$&90$07;3/*0$,4$.!/0&'*<$$L3*$(0*/,"*0$ *2*&0+$+3@9&$(0+!3"$*&!5;0-$90$7,"7;'#0#$&9!&$ &,$&90$07;3/*0$*9,:0#$9,:$+'79$90$9!#$;0!("0#$*3"70$ b$ &90$ (3"@*$ +'*&$ 7,"*3*&$ ,4$ #3*7,""07&0#$ /!(&37;0*$ b$ &90$*'/0(",1!$!5,'&$9,:$&,$#0!;$:3&9$*&!(*$!"#$,5607&*$ c503"@d$ 03&90($ *,;3#$ ,($ ;3O'3#-$ 5'&$ &902$ +'*&$ 50$
32 James Bryan E.E. Barnard and the 1889 Eclipse of Iapetus
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Barnard and the 1889 Eclipse of Iapetus credible. His ring theory was well known, but his /LQHRIVLJKW RSWLFDO GHSWK LV UHODWHG WR FKDQJH LQ bombardment hypothesis was effectively unknown. magnitude (ûP E\WKHDSSUR[LPDWLRQ Given that Barnard believed the particulate ring theory, 2 = ûP it is likely that he would have believed, or at least considered, equatorial bombardment, if he had known In the shadows of such features, Iapetus should have about it. In that case, he might have interpreted what IDGHGE\DSSUR[LPDWHO\DQGPDJQLWXGH he saw before the C ring eclipse as Iapetus in the 7KLVZRXOGKDYHEHHQDQRQHYHQWIRU%DUQDUG 37 James Bryan E.E. Barnard and the 1889 Eclipse of Iapetus %DUQDUG¶VUHVSRQVHWRFRORUPD\QRWKDYHEHHQDIDFWRU 7KHDUUDQJHPHQWRIVWDUVLQDILHOGRIYLHZFDQDIIHFW VLQFH WKH WKUHH VDWHOOLWHV KDYH DERXW WKH VDPH FRORU D YLVXDO REVHUYHU¶V VHQVH RI WKHLU EULJKWQHVV :KHQ &R[ UHSRUWHG PHDQ FRORU LQGLFHV LQ WKH UDQJH WZR VWDUV RI HTXDO EULJKWQHVV DQG VLPLODU FRORU DUH %9)RU,DSHWXVLQRUELWDOSKDVHV DUUDQJHG RQH DERYH WKH RWKHU ³« WKH ORZHU ZLOO %9 0LOOLV )RU7HWK\V DSSHDU WKH EULJKWHU SHUKDSV E\ DV PXFK DV KDOI D LQRUELWDOSKDVHV %9 PDJQLWXGH´ ,VOHV DQG /HZLV 7KH %ODLUDQG2ZHQ 6LPLODUUDQJHVIRU(QFHODGXV HIIHFW LV NQRZQ DV SRVLWLRQ DQJOH HUURU %DUQDUG ZHUHQRWDYDLODEOH SUREDEO\ NQHZ QRWKLQJ DERXW LW $Q REVHUYHU FDQ +H H[SHFWHG WR REWDLQ D IHDWXUHOHVV OLJKW FXUYH IRU DYRLG WKH SUREOHP E\ DUUDQJLQJ SDLUV RI VWDUV ,DSHWXV LQ WKH JDS EHWZHHQ SODQHW DQG ULQJV DV KH KRUL]RQWDOO\,WLVQRWSRVVLEOHWREHFHUWDLQKRZ%DU LPSOLHGZKHQKHZURWH³«WKHFXUYHVKRXOGEHIODWWHU QDUG RULHQWHG WKH WKUHH VDWHOOLWHV +LV QRWHV GR QRW QHDU K P WR FRUUHVSRQG ZLWK WKDW QHDU K P´ GLVFXVVWKLVEXWKLVVNHWFKRIWKHVFHQHLPSOLHVWKDWKH %DUQDUG 7KLV ZDV DQ DVVXPSWLRQ QRW D NHSW ,DSHWXV ORZ UHODWLYH WR 7HWK\V DQG (QFHODGXV IDFW +LV DQWLFLSDWLRQ RI HYHQWV FRXOG KDYH EHHQ D WKURXJKRXWWKHHFOLSVH:KHQ,DSHWXVDUULYHGDW SUREOHPVLQFHYLVXDOREVHUYHUVDUHNQRZQWRVHHZKDW SODQHWDU\ UDGLL LW UHDFKHG PD[LPXP EULJKWQHVV ³,W WKH\ EHOLHYH Figure 8: High resolution transmission curve of the C ring. Superimposed upon the modern transmission curve are normal optical depths (solid squares) derived from each of Barnard's visual magnitude estimates of Iapetus in eclipse. Squares indicate the center of Iapetus at the time of each of his observations. In the first half of the C ring eclipse Barnard's results agree with modern values. In the second half, his results are consistently greater than modern values. Uncertainty is about ± 0.02 normal optical depth based on an uncertainty of ± 0.1 magnitude in his estimates. Iapetus was large relative to the C ring's fine ringlets and plateaus which explains why Barnard did not resolve these features. (Transmission curve reprinted with permission of R.G. French and Elsevier from "Geometry of the Saturn system from the 3 July 1989 occultation of 28 Sgr and Voyager observations" by R.G. French et al., Icarus, 103, 163-214, 1993, figure 4, "Atlas of ring feature designations," copyright 1993 Elsevier.) 38 James Bryan E.E. Barnard and the 1889 Eclipse of Iapetus Figure 9: Low resolution transmission curve for the C ring. The plotted curve represents observations by Voyager 2. Filled circles are Pioneer 11 results. Filled squares are means of four normal optical depths as derived from Barnard's visual magnitude estimates of Iapetus in eclipse. Barnard correctly identified the faint C-D ring boundary at 1.235 planetary radii. He also correctly concluded that there is no gap at the B-C ring boundary at 1.525 planetary radii. He agreed with the general trend of modern transmission values in the inner C ring but disagreed in the outer C ring and D ring. Is this disagreement due to observational error or is it evidence for a condition that was not present for Pioneer 11 and Voyager 2? (Transmission curves for Voyager 2 and Pioneer 11 reprinted with permission of B.R. Sandel and Science from "Extreme ultraviolet observations from the Voyager 2 encounter with Saturn" by B.R. Sandel et al., Science, 215, 548-553, 1982, figure 4, "Normal optical depths in the C ring determined by the UVS during the delta Sco exit of the rings," copyright 1982 Science.) 7 BARNARD AND THE C RING ERXQGDU\DQGWKH%&ULQJERXQGDU\$VWRWKHODWWHU ,WZRXOGKHOSWRFRPSDUH%DUQDUG¶VUHVXOWVZLWKWKRVH VLQFHWKHVVRPHREVHUYHUVDUJXHGWKDWDGLYLVLRQ RI RWKHU REVHUYHUV EXW WKHUH DUH QRQH 7KHUH DUH H[LVWHGEHWZHHQWKH%DQG&ULQJV $OH[DQGHU KRZHYHUPRGHUQREVHUYDWLRQVRIRSWLFDOGHSWKLQWKH %DUQDUG JDYHKLVRSLQLRQ &ULQJ7RFRPSDUH%DUQDUG¶VREVHUYDWLRQVWRZKDWLV 1R GLYLVLRQ ZDV VHHQ EHWZHHQ WKH &UDSH ULQJ DQG WKH NQRZQDERXWWKH&ULQJWRGD\UHTXLUHVWKHDVVXPSWLRQ LQQHU EULJKW ULQJ DV KDV VRPHWLPHV EHHQ VKRZQ RQ WKDWWKHYLVXDODSSHDUDQFHRIWKHULQJKDVQRWFKDQJHG GUDZLQJV7KLVVXSSRVHGGLYLVLRQKRZHYHUZDVSURY VLJQLILFDQWO\LQPRUHWKDQDFHQWXU\,FRQYHUWHG%DU HGWRKDYHQRUHDOH[LVWHQFHE\P\REVHUYDWLRQVRIWKH QDUG¶VYLVXDOPDJQLWXGHGLIIHUHQFHVWRGLIIHUHQFHVLQ9 HFOLSVH RI -DSHWXV LQ WKH VKDGRZV RI WKH ULQJV PDJQLWXGHDQGIXUWKHUFRQYHUWHGWKHVHWRRSWLFDOGHSWK 1RYHPEHU LQWKHOLQHRIVLJKWDQGILQDOO\WRRSWLFDOGHSWKQRUPDO 39 James Bryan E.E. Barnard and the 1889 Eclipse of Iapetus Consequently, there is nothing to compare to Barnard ,DSHWXVDVGLIIHUHQFHVLQPDJQLWXGH,WUHODWHV,DSHWXV¶ within this boundary. The C ring is about 17,500 km observed magnitude during eclipse to its magnitude wide. Barnard agrees with Voyager 2 and Pioneer 11 outside of eclipse and to predicted magnitudes based over the first 9,000 km, from 1.235 to about 1.384 on modern optical depths for the C ring from 1.485 to planetary radii. Agreement means only that he antici- 1.517 planetary radii. It also compares changing Iapet- pated the general trend of normal optical depth as us to unchanging Enceladus. At 1.485 planetary radii measured by the spacecrafts. However, elsewhere Barnard estimated that Iapetus and Enceladus were there is disagreement. From 1.074 to 1.235 planetary equally bright. That is relevant for two reasons: the radii his optical depths are greater than can be ex- effect of position angle error on his estimates, and, plained by the modern D ring. That he was accurate at most importantly, the implication of modern optical this point must be inferred from his corroborated GHSWKV%DUQDUGLGHQWLILHG,DSHWXV¶SHDNYLVXDOPDJQL results in the nearby C ring where, from 1.235 to about tude at about 10.6. I correct that to V = 10.9 and apply 1.28 planetary radii, he obtained similar intensities. the difference of 0.3 magnitude to his other estimates Another disagreement occurs from about 1.384 planet- as a uniform correction for likely position angle error. DU\ UDGLL WR WKH %& ULQJ ERXQGDU\ ZKHUH %DUQDUG¶V Iapetus at V = 10.9 was about 0.9 magnitude brighter values dramatically diverge into greater densities. He than Enceladus at V = 11.8. For the range 1.485 to did not recognize this because he could not distinguish 1.517 planetary radii, based on Pioneer 11 in Figure 6, normal from abnormal densities in the C ring. His the general trend of normal optical depth increased last nine positive magnitude estimates were made in from about 0.07 to 0.12. Alternatively, Figure 5 shows the range 1.485 to 1.517 planetary radii when Iapetus approximately 0.04 to 0.14. Assuming that modern approached and then entered the B-C ring boundary. optical depths are relevant to the C ring in 1889, at These observations are significant because he saw 1.485 planetary radii, Barnard should not have seen Iapetus become fainter than Enceladus. That should equality. Iapetus was probably already fainter than not have happened as early as he saw it. Enceladus, with an approximate visual magnitude of 12.1. Only position angle error made it seem as bright as Enceladus. According to modern optical depths and without the effect of position angle error, he should have seen Iapetus to be brighter than Enceladus by 0.4 to 0.7 magnitude. The only moments during the C ring eclipse at which the two satellites should have been nearly equal were his last two observations at 1.515 and 1.517 planetary radii, very near the B-C ring boundary. However, by 1.517 planetary radii, despite his apparent over-estimates of its brightness, he still recorded Iapetus as about 0.8 magnitude fainter than Enceladus. At an opening angle of -8.49°, an increase in normal optical depth of about 0.11 could have affected Iapetus in this way. Modern optical depths imply that Barnard observed the C ring to be unusually dense over a large radial distance. If the ring has not significantly changed since 1889, another possibility is that something transitory affected it simultaneously with the eclipse. Showalter (1998) observed transient clumps in Sat- XUQ¶V)ULQJ7KHVHDSSHDUHGXQH[SHFWHGO\DQGIDGHG Figure 10: The plot compares Barnard's last nine differential in brightness over about two weeks. He interpreted magnitudes for Iapetus in eclipse to predicted differential WKHP WR EH ³« EXUVW HYHQWV «´ DQG VXJJHVWHG WKDW magnitudes for the satellite based on modern optical depths for the C ring from 1.485 to 1.517 planetary radii. The they are caused by high-speed impacts of approxi- unchanging magnitude of Enceladus (V = 11.8) is indicated for mately 10 cm-diameter meteoroids on ring bodies. further comparison. Zero on the differential magnitude axis Alternatively, burst events may be produced by corresponds to Iapetus' magnitude outside eclipse (V = 10.9). relatively slow collisions among rubble pile moons Barnard saw Iapetus equal Enceladus at 1.485 and 1.487 (Barbara and Esposito, 2002). planetary radii. Thereafter, Iapetus became fainter than Enceladus until he saw it disappear into the B ring's shadow. If With the equation the C ring's transmission characteristics in 1889 were as they are today, Iapetus should have equalled Enceladus' A' / A = (S r2 N1/3) / S R2, (5) magnitude only near Barnard's last two observations at 1.515 and 1.517 planetary radii. As a complication, his magnitude it can be shown that a cloud of particles from the estimates of Iapetus appear to be systematically too bright by destruction of a small moon could produce a shadow about 0.3 magnitude due to position angle error, a fault WKDW ZRXOG UHGXFH ,DSHWXV¶ PDJQLWXGH E\ DV peculiar to visual magnitude estimates in specific circum- stances. Even with an apparent tendency to over-estimate its Barnard observed just before the satellite entered the brightness, Barnard saw Iapetus become fainter than shadow of the C ring. Where A'/A is the ratio of the Enceladus too early. The C ring in 1889 might have been area obscured by the particle cloud to the area of visually different than it is today, but another possibility is that Iapetus, r is the radius of the disrupted moon, R is the spokes were present in the C ring at the time of the eclipse. At the B-C ring boundary the difference between Barnard's radius of Iapetus, and N is the number of particles results and modern normal optical depths is about 0.1 which is created in the burst event, the equation shows that for 10 similar to the observed density of spokes. moons with radii of 5 and 10 km, 2.4 × 10 and 3.8 × 108 particles, respectively, spread across the area of Figure 10 presents his last nine positive estimates of Iapetus (1.6 × 106 km2 ZRXOGFDXVHREVFXUDWLRQ 40 James Bryan E.E. Barnard and the 1889 Eclipse of Iapetus However, even if objects with radii of 5 to 10 km exist «WKDWWKHDEVHQFHRIVSRNHV>HDUOLHULQWKHPLVVLRQ@LV in the outer D ring, this scenario does not explain Bar- due to a seasonal modulation of the plasma environment QDUG¶V VLWXDWLRQ :KDW KH REVHUYHG LQYROYHG D YHU\ LQWKHULQJV7KHSKRWRHOHFWURQGHQVLW\DERYHWKHULQJV PXFKODUJHUDUHD5LQJVSRNHVFDQFRYHUODUJHDUHDV LV GHWHUPLQHG E\ VRODU LUUDGLDQFH KHQFH WKH HOHYDWLRQ DQJOHRIWKH6XQ 8 RING SPOKES 3RUFRHWDO IXUWKHUGHVFULEHGWKHSODVPD Spokes are tenuous, dark, ephemeral objects that HQYLURQPHQW DSSHDUWREH³«FRQILQHGWRWKHFHQWUDO%ULQJZLWKDQ +LJK 6XQ FUHDWHV D OD\HU RI SKRWRHOHFWURQV DERYH WKH LQQHUERXQGDU\DW«>SODQHWDU\UDGLL@DQG ULQJV WKDW FDQ QHJDWLYHO\ FKDUJH VPDOO GXVW SDUWLFOHV DQRXWHUERXQGDU\DWDSSUR[LPDWHO\WKHRXWHUHGJHRI DERYH WKH ULQJV SXOOLQJ WKHP EDFN WR WKH SRVLWLYHO\ WKH%ULQJ´ 6PLWKHWDO 0D[LPXPUDGLDO FKDUJHG ULQJV $ ORZ « >6XQ@ DQJOH UHGXFHV WKH « DQG D]LPXWKDO GLPHQVLRQV RI DQG NP SKRWRHOHFWURQOD\HUFDXVLQJGXVWSDUWLFOHVWRKDYHDQHW UHVSHFWLYHO\ KDYH EHHQ REVHUYHG EXW QDUURZ DQG ]HUR RU VOLJKWO\ SRVLWLYH FKDUJH DQG WKHUHIRUH WR EH ILODPHQWDU\ VKDSHV DOVR RFFXU *UQ HW DO UHSHOOHGE\WKH«ULQJ7KHUHODWLYHO\KLJK6XQHOHYD 7KH\ DUH GDUN LQ EDFNVFDWWHUHG OLJKW DQG EULJKW LQ WLRQDWSUHVHQWPD\FUHDWHDQHQYLURQPHQWKRVWLOHWRWKH IRUZDUGVFDWWHUZKLFKLQGLFDWHVWKDWWKH\FRQVLVWRIILQH DSSHDUDQFHRIVSRNHV GXVW 6SRNHVDSSHDU DW DQ\D]LPXWK RQWKH ULQJV EXW 0LWFKHOOHWDO DQWLFLSDWHGWKDWVSRNHVZLOO PRVWRIWHQDWWKHHDVWHUQRUPRUQLQJDQVD7KH\ODVW EH VHHQ LQ PLG WR ODWH ³« LI WKH SODVPD FRQ for about one-fourth to one-third of the orbital period ditions are favorable for their formation and either the RIWKHPDJQHWLFILHOG GH3DWHUDQG/LVVDXHU REVHUYHURUWKH6XQLVQHDUWKHULQJSODQH´ KRXUV PLQXWHV7KHLUW\SLFDOQRUPDORSWLFDOGHSWK LVDERXW *UQHWDO %DUQDUG¶VREVHUYDWLRQLVFRQVLVWHQWZLWKWKUHHSURS HUWLHV RI VSRNHV )LUVW VSRNHV DUH EHVW VHHQ DW ORZ 7KHUH LV QR FRQVHQVXV IRU WKH FDXVH RI VSRNHV GH RSHQLQJ RU VRODU LOOXPLQDWLRQ DQJOHV 0F*KHH HW DO 3DWHUDQG/LVVDXHU KRZHYHUWKH\DUHWKRXJKWWR IRXQG WKDW ³« D UHODWLYHO\ ORZ RSWLFDO be charged dust particles with a size of a micrometer or GHSWK«LVVXIILFLHQWWRSURGXFHWKHREVHUYHGFRQWUDVW less that are levitated over the rings through electro- >EHWZHHQ VSRNHV DQG WKHLU VXUURXQGLQJV@ «´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¶VDQJXODUYHORFLW\DVGHILQHG curve identifies what could be interpreted as two radi- E\WKHURWDWLRQDOSHULRGRIWKHSODQHW¶VPDJQHWLFILHOG DOO\ H[WHQGHG IHDWXUHV WKDW ZHUH VXSHULPSRVHG XSRQ &DURO\Q 3RUFR DQG (GZDUG 'DQLHOVRQ DQG WKH%&DQG'ULQJV7KH'ULQJIHDWXUHKDGDUDGLDO 3RUFR IRXQGWKDWFKDQJHVLQWKHDSSHDUDQFHRI H[WHQWRIDERXW NP7KHRUELWDOSHULRGLVDERXW spokes are correlated with the orbital periods of the KRXUV LQ WKH PLGGOH ' ULQJ 6LQFH ,DSHWXV WRRN magnetic field and of broadband radio emissions called DERXW PLQXWHV WR WUDQVLW WKH IHDWXUH VSRNHV ZRXOG 6DWXUQ (OHFWURVWDWLF 'LVFKDUJHV (YHQ VR &KULVWRSK KDYHKDGWRH[WHQGDERXWRU NPDORQJWKH *RHUW] DQG *UHJRU 0RUILOO XUJHG WKDW JUDYLW\ DUF RI WKH ULQJ 7KH & ULQJ IHDWXUH FRYHUHG DW OHDVW not electromagnetic force, dominates the motion of NPDQGFRQWLQXHGLQWRWKH%ULQJ$WDUDGLXVRI ULQJGXVWSDUWLFOHV7KH\SURSRVHGWKDWGHQVHSODVPD NP WKH RUELWDO SHULRG LV DERXW KRXUV columns are created as meteoroids impact ring bodies ,DSHWXV WRRN DERXW PLQXWHV WR WUDQVLW WKH IHDWXUH DQG WKDW WKHVH FROXPQV HYHQWXDOO\ FRURWDWH ZLWK WKH 6SRNHVZRXOGKDYHKDGWRH[WHQGDERXWRU SODQHW¶VPDJQHWLFILHOG&KDUJHGGXVWSDUWLFOHVLQWKH NPDORQJWKHDUFRIWKH&ULQJ7KLUGO\VSRNHVKDYH SODVPD FORXG DUH HOHFWURVWDWLFDOO\ H[SHOOHG IURP WKHLU QRUPDORSWLFDOGHSWKVRIDERXW:KDW%DUQDUGVDZ resting places in the ring when the electric force KDGDQRUPDORSWLFDOGHSWKRIDERXWDWLWVGHQVHVW EHFRPHVVWURQJHUWKDQJUDYLW\&ROOHHQ0F*KHHHWDO SRLQW RQ WKH %& ULQJ ERXQGDU\ 1HYHUWKHOHVV WR H[DPLQHGWKHSKRWRPHWULFSURSHUWLHV H[SODLQKLVREVHUYDWLRQZLWKVSRNHVLVXQFRQYHQWLRQDO of spokes as recorded in Hubble Space Telescope 3ODQHWDU\VFLHQWLVWVEHOLHYHWKDWWKH\DUHOLPLWHGWRWKH +67 LPDJHVIURP WR6SRNHVZHUHYLVLEOH FHQWUDO%ULQJ on either side of ring plane crossing but became fewer DQG IDLQWHU XQWLO QR VSRNHV ZHUH REVHUYHG EH\RQG DQ (YLGHQFHIRUVSRNHVLQWKHB ring and for spoke-like RSHQLQJ DQJOH RI $IWHU PRGHOLQJ DOWHUQDWLYH IHDWXUHV LQ WKH $ DQG & ULQJV KDV EHHQ FROOHFWHG E\ DUUDQJHPHQWV RI GXVW UHODWLYH WR WKH ULQJV WKH\ YLVXDOREVHUYHUVIRUDORQJWLPH6WHSKHQ-2¶0HDUD FRQFOXGHG WKDW ³« WKH VWURQJ WLOW HIIHFW RQ VSRNH LV WKH PRVW VXFFHVVIXO YLVXDO REVHUYHU RI VSRNHV FRQWUDVW FDQ EH DFFRXQWHG IRU DV D UHVXOW RI YDU\LQJ %HJLQQLQJLQ KHXVHG DQGLQFKUHIUDFWRUV YLHZLQJ DQG LOOXPLQDWLRQ JHRPHWU\ RI DQ H[WHQGHG WRYLVXDOO\HVWLPDWHPDJQLWXGHD]LPXWKDOYDULDWLRQV OD\HURIGXVWWKDWOLHVDERYHWKHULQJLWVHOI´$OWKRXJK LQWKHEULJKWQHVVRIWKH$ULQJ2EVHUYLQJLQWZLOLJKW WKH\SUHGLFWHGWKDW³«VSRNHVVKRXOGEHHDVLO\GHWHFW WR GLPLQLVK WKH DSSDUHQW EULJKWQHVV RI WKH $ ULQJ KH able during the Cassini mission when the rings are XQH[SHFWHGO\IRXQGGDUNUDGLDOIHDWXUHVLQWKH%ULQJ YLHZHG DW UHODWLYHO\ VPDOO B ULQJ RSHQLQJ These had the rotational period of the planet and did DQJOHV «´ LW WRRN IURP -XO\ ZKHQ &DVVLQL QRW H[KLELW .HSOHULDQ PRWLRQ 7KH\ WHQGHG WR SUHIHU DFKLHYHGRUELWXQWLO6HSWHPEHUEHIRUHWKHVSDFH WKHPRUQLQJDQVDDQGWKHLUYLVLELOLW\ZDVUHODWHGWRULQJ FUDIW REVHUYHG VSRNHV 7KHVH DSSHDUHG RQ WKH GDUN RSHQLQJDQJOH +LV UHSRUWVZHUH GLVEHOLHYHGDQGKLV side of the rings when the angle to the spacecraft was DWWHPSWV WR SXEOLVK ZHUH UHIXVHG $IWHU WKH 6DWXUQ 0LWFKHOOHWDO &ROLQ0LWFKHOODQG &RQIHUHQFHLQ7XFVRQ$UL]RQDLQ0D\ 2¶0HDUD 0LKDO\+RUiQ\L SURSRVHG was recognized for his results, but these made no 41 James Bryan E.E. Barnard and the 1889 Eclipse of Iapetus lasting impression outside amateur astronomy even « WKH VHUUDWHG DSSHDUDQFH RI WKLV >$@ ULQJ ZKHUH LW when events were fresh: ERUGHUV&DVVLQL¶VGLYLVLRQZDVVHHQZLWKJUHDWFHUWDLQW\ Visual observers have also occasionally claimed to see RQVHYHUDOQLJKWV«>2Q@-XQHWKHLQQHUHGJHRIULQJ transient, dark radial features and bright spots in the $ORRNHG«VKDUS2Q-XO\VRPHYHU\FRQVSLFXRXV rings (Alexander 1962). These reports are especially VHUUDWLRQV ZHUH VHHQ RQ WKH I DQVD « 2Q 0D\ WKH intriguing in the light of Voyager discovery of spokes LQQHUHGJHRIWKHSDQVD>RIULQJ&@DSSHDUHGVHUUDWHG « KRZHYHU OLNH PDQ\ RWKHU YLVXDO UHSRUWV WKH\ DUH VRPHZKDWOLNHWKDWRIULQJ$« difficult to assess objectively. (Cuzzi et al., 1984: 75). Rev. T. H. Foulkes (Green, 1897: 237) left this record: :KDW2¶0HDUDGLGHQWLWOHGKLPWRGLVFRYHU\FUHGLWEXW 1RWLFHGDUHPDUNDEOHDSSHDUDQFHRIWKH>&@ULQJZKHUH historically others came close to that distinction. it crossed the ball, it did not possess its usual uniform DSSHDUDQFHEXW ZDV GHFLGHGO\ µOXPS\¶ , FRXQWHGVL[ 9 HISTORICAL OBSERVERS AND O’MEARA or seven of these darker shadings, which seemed to KDYHDWHQGHQF\WRFLUFXODUIRUPDWLRQ«7KRXJK«>, %DUQDUGGLGQRWEHOLHYHWKDWGDUNIHDWXUHVRQ6DWXUQ¶V KDYHREVHUYHG6DWXUQ@IRUWKHODVW\HDUV,KDYHQHYHU rings were real. Even so, his result for the eclipse of before seen this curious formation. Iapetus appears to be consistent with claimed activity +RZHYHU )RXONHV¶ REVHUYDWLRQ PD\ EH XQUHODWHG WR in the C ring in the years around 1889. It was a routine WKH & ULQJ ,Q 5LFKDUG 0F.LP SHUV FRPP matter for nineteenth-century observers to describe 2005) and others saw similar clumping in the C ring. DSSDUHQW FKDQJHV LQ 6DWXUQ¶V ULQJV %\ WKH PLGGOH They did not suspect changes in the ring. Instead, they WZHQWLHWK FHQWXU\ KRZHYHU $OH[DQGHU¶V FRP preferred the possibility that dark spots in the North prehensive analysis of the historical record explained Equatorial Belt, which lay beneath the ring, were many unusual claims as unreal effects created by visible through it. An observation by the Cassini illusion and error. While Alexander made valid points, spacecraft on 28 April 2006 appears to justify that he did not have the advantage of knowing that spokes LQWHUSUHWDWLRQ %ULJKW FORXGV LQ WKH SODQHW¶V DWRP in the B ring are real. Further, the Cassini-Huygens sphere were only partially obscured by the shadow of mission makes clear that spokes are difficult to ex- the C ring (NASA/JPL/SSI, 2006). plain. Interesting and perhaps significant historical examples of reported change in the rings include the Barnard (1895: 369-371), who saw none of this, was following episodes. not persuaded. He treated the matter with sarcasm: Trouvelot (1877: 191) identified spoke-like features The Crape ring has appeared uniformly even in shade at in what he called the B ring but that is now known as the two ansae. It was of a steelly blue colour, and the A ring: was not strongly contrasted with the sky. No markings whatever were seen upon it. The inner edge was a «WKHLQQHUPDUJLQRIWKH>$@ULQJ«OLPLWLQJWKHRXWHU uniform curve; the serrated or saw-toothed appearance ERUGHURIWKHSULQFLSDO>&DVVLQL@GLYLVLRQKDVVKRZQRQ of its inner edge, which had previously been seen with WKHDQVDHVRPHVLQJXODUGDUNDQJXODUIRUPV«WKHVXU VRPHVPDOOWHOHVFRSHVZDV«EH\RQGWKHUHDFKRIWKH IDFH RI WKH >$ DQG RXWHU %@ ULQJV « KDV VKRZQ D 36-inch. mottled or cloudy appearance on the ansae during the ODVWIRXU\HDUV« Considering the vivid drawings of Saturn that others produced, Barnard was unapologetic for his own art François J.C. Terby (1887: 163) announced the pres- WKDW³«DSSHDUVDEQRUPDOO\GHYRLGRIGHWDLOV«,DP HQFHRI³«PDVVHVVRPEUHVGDQVO¶DQQHDXREVFXU«´ satisfied, however, to let it UHPDLQVR´+LVDVVHVVPHQW (big dark blotches in the dusky ring). Not everyone of results by George Davidson, a San Francisco am- was convinced, but Thomas G.E. Elger (1887: 512) DWHXUDVWURQRPHUFRXOGKDYHDSSOLHGWRKLVRZQ ³2QH was emphatic about their reality: great thing must commend itself to every observer 26th)HEUXDU\>@«WKHS>UHFHGLQJ@DQVD>RIWKH& familiar with Saturn in a telescope is that he has not ULQJ@H[KLELWVRQLWVLQQHUERUGHUWKUHHRUIRXUODUJHUH VKRZQ D VLQJOH DEQRUPDO IHDWXUH´ $OWKRXJK KH entering angles like the teeth of a saw, the intervening apparently did not know it, his situation was not so spaces being apparently as dark as that between the ball simple. and the ring, and extending nearly to the outer edge of the ring. 12th0DUFK>@«SDQVDLVYHU\HYLGHQWO\ On 7 January 1888, the night of the first successful broken up into several areas of different degrees of WHVWRIWKHLQFKUHIUDFWRU-DPHV(.HHOHUGUHZDQ darkness, so that, except a short section of it, np, it is LPDJH RI 6DWXUQ WKDW EHFDPH NQRZQ DV ³« WKH EHVW impossible to recognise it as a ring surrounding the H[LVWLQJSLFWXUHRIWKHSODQHWIRUPDQ\\HDUVDQG>WKDW@ SODQHW7KHI>ROORZLQJ@DQVD«LVHDVLO\YLVLEOH was widely admired by professional astronomers of the In April 1890, about six months after the eclipse of WLPH«´ 2VWHUEURFNDQG&UXLNVKDQN $V Iapetus when the rings were slightly more open, Paul .HHOHUGUHZLWWKH%ULQJKDGWKUHHIDLQWGDUNGLIIXVH Stroobant (1890) observed dark notches with puzzling radial shapes upon it that suggest spokes. The shapes shapes on the inner edge of the evening ansa of the C are not obvious and are only noticeable as departures ULQJ,Q$SULO (XJqQH0$QWRQLDGL from circularity within thH ULQJ ,W ZDV %DUQDUG¶V UHSRUWHG ³« LQVWHDG RI WKH (QFNH GLYLVLRQ ULQJ $ RSLQLRQ WKDW .HHOHU KDG DUWLVWLF DELOLW\ WKDW IHZ RWKHU shows (just now) some enormous white spots separated observers possessed (Sheehan, 1995: 149). There is by dusky intervals. This ring appears broken (as it every reason to suppose that he drew the B ring just as ZHUH LQWRIUDJPHQWV´,Q-XQHKHDGGHGWKDWWKH³« he saw it, including the likeness to faint spokes. >$@ ULQJ VKRZHG LWVHOI ODWHO\ FRPSRVHG RI VXFFHVVLYH 6KHHKDQ DOVRWKRXJKWWKDWKLVGUDZLQJ³« groups of white spots, separated by dusky intervals, JLYHV KLQWV RI WKH µVSRNHV¶ RQ WKH VXUIDFH RI 5LQJ % which seemed to shoot forth in the direction of radii «´.HHOHUHYHQWXDOO\SUHVHQWHGDFRS\RIKLVIDPRXV HPDQDWLQJIURP6DWXUQ¶VFHQWUH´ *UHHQ drawing to Barnard as a gift (Osterbrock and Others saw similar effects. Charles Roberts (Green, Cruikshank, 1983). In receiving it, Barnard had 1897: 244) reported that HYLGHQFHWKDWRGGIHDWXUHVRQ6DWXUQ¶VULQJVZHUHZHOO 42 James Bryan E.E. Barnard and the 1889 Eclipse of Iapetus within reach of the 36-inch refractor. Surviving letters written by Keeler to Barnard, now collected at Vanderbilt University, do not discuss the Saturn GUDZLQJ6LQFH.HHOHU¶VREserving record for January 1888 is lost, it is not possible to know what he thought about the appearance of the B ring. +LVWRULFDO REVHUYHUV WHQG WR GLIIHU ZLWK 2¶0HDUD over the intensity of transitory dark ring features. Elger is a good example. He insisted upon the ob- viousness of these objects and drew them vividly )LJXUH 2¶0HDUD SHUVFRPP HPSKDVL]HG Figure 11: Engraved drawings of the C ring from March 1888. their subtle appearance, but drew them just as vividly In 1887 and 1888 Thomas Elger was one of several observers )LJXUH who reported unusual dark markings on the C ring. Their existence was controversial. Elger (1888) described what he ,¶YH QHYHU VHHQ EODFN PDUNLQJV RU JDXJHV $QG ,¶P saw as follows: "March 21 … Inner edge ragged and clearly certain no one in history has (not even Elger). You indented on p. side, but indentations not very deep … March KDYH WR FRQVLGHU WKH DUWLVW¶V VW\OH ZKHQ KH RU VKH LV 27 … Inner edge on p. ansa scalloped, exhibiting three or four trying to portray a dim feature. For instance, even in convex projections, and dark patches visible on its surface." my drawings, the spokes look very intense, but they are South is up in these images. If the engravings are faithful to not. They are definite but delicate to the eye, very hard Elger's original drawings, the dark features had an angular extent along the ring that approached 45°. Taking into account to render in a way that will reproduce, unless you rotation rates in the C and D rings, what Barnard observed 19 intensify them. months later during the eclipse of Iapetus must have had a Elger was an experienced lunar and planetary similar extent to have affected his observation as it did. (Reprinted from "Physical observations of Saturn in 1888" by observer, and it is certainly possible that he meant T.G. Elger, MNRAS, 48, 362-370, 1888, figure entitled "Ring exactly what he wrote. The degree to which something C, March 21 [and] Ring C, March 27" with permission from is obvious depends, after all, on the observer. How- Blackwell Publishing.) ever, Elger had critics who saw no dark features. They suggested that either his equipment or his eyesight Is the visibility of spokes limited to small ring open- was faulty. Elger (1888) replied that only dabblers in LQJDQJOHV"0F*KHHHWDO GHWHFWHGQRVSRNHV Saturn could fail to see what he reported. Disagree- EH\RQGDQRSHQLQJDQJOHRI,Q$XJXVW ment that declined into personal jabs makes it possible, ZKHQWKHDQJOHZDVDERXW2¶0HDUD SHUVFRPP even likely, that some of his emphasis was intended to REVHUYHGVSRNHVZLWKWKH0RXQW:LOVRQP defeat critics. Then differences over intensity may be telescope. He described their color as ice blue. In an effect of non-observational influences such as the $XJXVW KHXVHGWKHPWHOHVFRSHDW3LFGX0LGL need to create a reproducible illustration or the desire 2Q WKDW RFFDVLRQ ZKHQ WKH DQJOH ZDV DERXW KH to make a point. Not all the historical cases are so. described spokes as gray in color and stronger in $OWKRXJK .HHOHU DSSDUHQWO\ PDGH QR FODLP IRU GDUN DSSHDUDQFHWKDQWKH\KDGEHHQLQWKHP3UHVXP features, his drawing is consistent with tenuous spokes ably, changed opening angle affected his sense of the LQWKH%ULQJDV2¶0HDUDGHVFULEHG+RZHYHUYLVXDO color and contrast of spokes. In both cases opening observations, both historical and modern, disagree with DQJOHV ZHUH LQ H[FHVV RI 2QH RI KLV HDUOLHVW WZR SUHVHQWO\DFFHSWHG FRQGLWLRQV IRU VSRNHV²WKDW observations of spokes was made at an opening angle they occur only in the B ring, and that their visibility is RIDERXW 5RELQVRQ $VWRVPDOOHUDQJOHV limited to small ring opening or solar illumination KHVDZVSRNHVLQJUHDWHVWQXPEHUVIURP2FWREHU angles. WR0DUFK ZKHQWKHDQJOHFORVHGIURPDERXW WR +H VDZ IDU IHwer spokes from November 10 A DIFFERENT PERSPECTIVE WR-DQXDU\ DVWKHDQJOHFKDQJHGIURPDERXW $UH VSRNHV FRQILQHG WR WKHFHQWUDO% ULQJ" 2¶0HDUD WR+LVFRXQWLQFUHDVHGDJDLQIURP)HEUXDU\WR SHUV FRPP GHVFULEHG WKH $ ULQJ DV EHLQJ -XQH ZKHQ WKH DQJOH RSHQHG IURP DERXW WR SURQHWRHSKHPHUDOVKDGHGSDWFKHV+HKDVVHHQ³« 8QOLNH 0F*KHH HW DO ZKR FRQWLQXHG WR 5LQJ & DSSHDU SDWFK\ DW WLPHV LQ ODUJHU VFRSHV´ $W GHWHFW VSRNHV EHWZHHQ DQJOHV RI DQG YHU\ 3LFGX0LGLRQ$XJXVW KHVDZ³«WZRGDUN narrow viewing geometry was a disadvantage for UDGLDOIHDWXUHVRQWKHVRXWKHDVWTXDGUDQWRI5LQJ&² 2¶0HDUD QRWDWWKHDQVD«7KHSUHFHGLQJ>HYHQLQJ@DQVDORRNHG :K\ ZHUH 2¶0HDUD¶V UHVXOWV GLIIHUHQW" :KLOH WKH XQLIRUP´7KHVHIHDWXUHVZHUHD]LPXWKDOO\DVVRFLDWHG spoke process apparently has a time-scale of years that with but were not connected to five B ring spokes that is related to solar illumination of the rings, individual KDGDVDZWRRWKHGDQGFXUYHGDSSHDUDQFH spokes clearly exist and change on a time-scale of min- They were radial but certainly different than those in utes and hours. To observe spoke dynamical changes 5LQJ%«>EHLQJ@EURDGHUDQGPRUHOLQHDU«,«>UH ZLWK+67%UDGIRUG6PLWK DQWLFLSDWHG FDOO@WKHLUGLPQHVV7KHVSRNHVLQ5LQJ%ZHUHPXFK that orbital limitations would impose non-continuous PRUHREYLRXV«EHFDXVH«,ZDVORRNLQJDW DµGDUN¶ data sets so that VKDGLQJ DJDLQVW D EULJKW ULQJ « LQ 5LQJ & , ZDV RE VHUYLQJ µVOLJKWO\ GDUNHU¶ IHDWures against a relatively «ZHFDQQRWHVFDSH«WKHVDPHSUREOHPVHQFRXQWHUHG dark ring, which is much haUGHUWRGR«,KDGWRXVH with the Voyager images. Statistically, one can par- DYHUWHG YLVLRQ WR VHH WKHP « , EHOLHYH WKLV PLJKW EH WLDOO\ RYHUFRPH WKH SUREOHPV RI D REVHUYLQJ GXW\ ZK\%LOO>6KHHKDQZKRVDZWKHILYH%ULQJVSRNHV@GLG cycle by extending the total observing time. Typical not see the features in Ring C. VSRNHOLIHWLPHVDUHaKU«DQGWKXVWKHDFFXPXODWLRQ of several tens of hours of observing time by recording 6KHHKDQ SHUVFRPP GLG QRWORRNIRUVSRNHV WKHULQJVIRUPLQXWHVSHURUELWZRXOGOLNHO\\LHOG in the C ring. PDQ\ « >VSRNH HYHQWV@ « WKH 9R\DJHU GDWD KDYH 43 James Bryan E.E. Barnard and the 1889 Eclipse of Iapetus taught us that sporadic observations are of relatively 11 WHY NOT BARNARD? little value for dynamical studies. The ring system appears to have been visibly active in McGhee et al. (2005: 508) observed the planet with \HDUVEHIRUHDQGDIWHUWKHHFOLSVHRI,DSHWXV,IWKDWLV several hundred high resolution images obtained on 34 true, did Barnard see anything unusual on the rings GDWHV IURP WR 7KH\ LGHQWLILHG ³« during the eclipse? He described what he saw: spokes or spoke complexes, predominantly on the PRUQLQJ HDVW DQVD´ )URP WR 2¶0HDUD The superb definition of the planet in the last part of the observations showed no abnormal appearance of the observed several times each week during apparitions of rings where the shadow of the ball crosses them, nor WKHSODQHW5HSUHVHQWDWLYHRIKLVSDFH2¶0HDUD SHUV KDYH,DWDQ\WLPHVHHQDZKLWHVSRWRQWKHULQJVDWWKLV comm., 2005) observed 29 spokes in a period of 43 or any other point. (Barnard, 1890). GD\V IURP -DQXDU\ WR 0DUFK ,I JDSV LQ coverage on a time-scale of hours prevent correct The white spot Barnard referred to was a contrast understanding of spoke dynamics, gaps that range effect seen seven months earlier by Terby. He said to months must interfere with understanding other QRWKLQJ DERXW GDUN IHDWXUHV RQ WKH ULQJV ,I VSRNHV SURSHUWLHV7KDW2¶0HDUDUHSRUWHGVSRNHVRXWVLGHWKH were there, why did he not see them? B ring and at large ring opening angles while planetary Barnard left no indication that he anticipated any- scientists through January 2004 observed neither does WKLQJ RWKHU WKDQ DQ H[KLELWLRQ RIWKH& ULQJ¶V QRUPDO QRW QHFHVVDULO\ PHDQ WKDW KH ZDV ZURQJ ,W PD\ transmission characteristics, but it would not have been be that his rate of observation made a meaningful excessive for him to have wondered about possibilities. difference. The advantage of being first to observe the inner ring system in transmission was more than enough reason to anticipate something new. Yet his first report to the Astronomical Society of the Pacific was so narrowly GLUHFWHG WR 0DUWK¶V TXHVWLRQ DERXW WKH & ULQJ DV WR imply that he had only one purpose on his mind, so much so that he may have overlooked subtle spokes. How likely is that? Using a telescope of comparable VL]H2¶0HDUD SHUVFRPP IRXQGWKDWZKHQKLV attention was directed to ring divisions, he could not see spokes that were present on the rings. The con- verse was also true. Barnard observed with a magnification of 150×. 2¶0HDUD SHUVFRPP EHOLHYHVWKDWWKLVLVWRR low a power to distinguish spokes on the bright B ring, HVSHFLDOO\ DW QLJKW DV RSSRVHG WR GXULQJ WZLOLJKW ,Q 2¶0HDUD¶VFDVHîZDVSUREDEO\WKHPLQLPXPXVH IXO PDJQLILFDWLRQ ZLWK î WR î EHLQJ EHWWHU Because spokes are delicate and their surroundings are EULJKWKHRIWHQ³«ZRXOGREVHUYHRQO\RQHVLGHRIWKH ULQJV«DWDWLPH«´E\XVLQJWKHHGJHRIWKHILHOGDV DQRFFXOWLQJEDU+LVWRULFDOO\(OJHU ZRUNHGLQ WKHUDQJHîWRî$QWRQLDGLXVHGîWRî ZLWKDSUHIHUHQFHIRUî *UHHQ DQG6WURR EDQW XVHGî Another factor concerns what Barnard could see on planets. People respected and were sometimes amazed by his visual observations, but they were also vexed E\ ZKDW WKH\ WKRXJKW KH FRXOG QRW VHH %DUQDUG¶V successses and access to the ZRUOG¶VODUJHVWWHOHVFRSHV encouraged him to assume superior authority in DQVZHULQJREVHUYDWLRQDOTXHVWLRQV+HZDVQRWUHOXF tant to disappoint others over their claimed discoveries. Because those whom he contradicted included exper- Figure 12: Saturn as drawn by Stephen J. O'Meara on 1-2 ienced planetary observers, there was an inevitable August 1992 at the 1-m telescope of Pic du Midi Observatory. O'Meara saw spokes in the B ring. He also saw even fainter FRQVHTXHQFH $FFRUGLQJ WR $QWRQLDGL D ³« spoke-like features in the C ring, but in this image he %DUQDUGQ¶HVWSDVXQREVHUYDWHXUGHGpWDLOVSODQpWDLUHV exaggerated their density for clarity. O'Meara is not first to GpOLFDWV «´ >%DUQDUG LV QRW DQ REVHUYHU RI GHOLFDWH record such activity in the C ring. In the 1880s and 1890s SODQHWDU\ GHWDLOV@ 2WKHUV DJUHHG *LYHQ KLV UHFRUG astronomers argued the reality of claimed transitory dark features in that ring. Illustrative of the challenge that spokes did he really not see what others saw? present to visual observers, O'Meara put the planetside edge $QWRQLDGL FULWLFL]HG RQ WKH RFFDVLRQ RI %DUQDUG¶V of B ring spokes in contact with the B-C ring boundary. Other observers have done the same. However, spacecrafts show inability to see a fourth ring reported by Georges spokes in the central B ring that are not in contact with the B-C )RXUQLHUWREHMXVWRXWVLGH6DWXUQ¶V$ULQJ$QWRQLDGL ring boundary. Visual observers may not be able to distinguish (1909a: 450) sardonically observed that since the new the darkness of spokes from the darkness of the inner B ring ring leading to a perception that spokes reach the B-C ring boundary. As shown here, north is up (courtesy: Stephen J. «DpWpDEVROXPHQWLQYLVLEOHjO¶LOOXVWUHGpFRXYUHXU« O'Meara). RQ FRQYLHQGUD TX¶LO QH VDXUDLW SOXV rWUH TXHVWLRQ GH 44 James Bryan E.E. Barnard and the 1889 Eclipse of Iapetus O¶H[LVWHQFH G¶XQ DQQHDX H[WpULHXU FUpSXVFXODLUH GH REMHFWV :KLOH LW ZRXOG EH KHOSIXO RU HYHQ ILQDO LI 6DWXUQH>«LVDEVROXWHO\LQYLVLEOHWRWKHJUHDWGLVFRY %DUQDUGKDGVHHQZLVS\GDUNIHDWXUHVRQWKHPRUQLQJ HUHU « ZH ZLOO DJUHH WKHUH FDQ EH QR TXHVWLRQ RI VLGH RI 6DWXUQ¶V ULQJV GXULQJ WKH HFOLSVH WKDW KH VDZ ZKHWKHURUQRW6DWXUQ¶VH[WHULRUFUHSHULQJH[LVWV@ QRQH LV XQUHODWHG WR KLV DELOLW\ WR HVWLPDWH ,DSHWXV¶ :LWK VLPLODU ELWHKH DOOXGHG WR DQRWKHU GLVDJUHHPHQW PDJQLWXGH LQ HFOLSVH +LV UHFRUG GHPRQVWUDWHV WKDW LQ WKH V ZKHQ %DUQDUG FRXOG QRW VHH VSRWV LQ IDLQWVWDUVDQGVWDUOLNHREMHFWVZHUHOHVVRIDSUREOHP 6DWXUQ¶V DWPRVSKHUH WKDW ZHUH UHSRUWHG E\ $UWKXU IRU KLP WKDQ WKH\ ZHUH IRU RWKHUV %XUQKDP 6WDQOH\ :LOOLDPV ³« RQH RI WKH PRVW RXWVWDQGLQJ 6KHHKDQ QRQSURIHVVLRQDO DVWURQRPHUV RI PRGHUQ WLPHV «´ 2ELWXDU\ 7KDWGLVDJUHHPHQWZDVDV 12 EVALUATION AND CONCLUSION PXFKVRFLRORJLFDODVLWZDVREVHUYDWLRQDO3URIHVVLRQ DOV ZHUH UHSODFLQJ DPDWHXUV DV OHDGHUV LQ DVWURQRP\ 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$OWKRXJKLWZDVVXEVWDQWLDOO\FRUUHFWKLVUHVXOWZDVWRR 45 James Bryan E.E. Barnard and the 1889 Eclipse of Iapetus strange and too unsatisfactory to be believed before others to evaluate what he had seen. He never sug- Voyager 1. That outcome was either an understand- gested the existence of an unseen ring. He certainly able mistake or appropriate conservatism. Either way, did not associate his observation with controversial a fundamentally correct description of a previously- dark spots on the C ring for he publicly rejected their unknown phenomenon of the ring system was reality. He refused to believe that the first 30 of his 75 disbelieved. magnitude estimates of Iapetus revealed anything un- The prospects for Barnard after the eclipse of Iapetus known about the ring system. Did he sacrifice a ZHUHHQWLUHO\GLIIHUHQW8QOLNH2¶0HDUDKHZDVZHOO meaningful observation because it was unexpected, known as a planetary scientist. In 1889 his form of extraordinary, and not confirmable? Common sense photometry was generally accepted, his instrument- and his own considerable experience must have guided ation was suitable, and his application was novel. his decision, but as was true at other times, his fear of Nobody published doubt over the correctness of his ridicule may also have been at work. The conservative result and conclusion even though he alone saw the answer was safe, but did it downplay a real phenomen- HFOLSVH%DUQDUG¶VWKUHHSXEOLVKHGDFFRXQWVVKRZWKDW RQDVKDSSHQHGLQ2¶0HDUD¶VFDVH he was in a dilemma over interpretation of what he The conjecture that Barnard saw spoke shadows on VDZ (LWKHU WKH DQRPDORXV GHFOLQH LQ ,DSHWXV¶ PDJ Iapetus contradicts modern scientific understanding of nitude before the C ring eclipse was real or it was not. where spokes occur. It is otherwise consistent with the If he called it real, what explained it? normal optical depth of spokes, their increased contrast An interpretation that remained attractive until Pio- at small ring opening angles, and their ability to cover neer 11 visited Saturn in 1979 was that he had long radial distances and broad areas. It appears to be observed the shadow of an unseen ring interior to the C consistent with observations of transitory dark mark- ring. If he had made that claim, it would have been ings on the C ring in the late 1880s as well as with a plausible because the relatively recent discovery of the similar case in 1992. See Figure 13 for an observation C ring made the existence of another tenuous interior made by Stroobant a few months after the eclipse of ring believable. It would have contradicted no estab- Iapetus. However, even if this interpretation is wrong, lished fact about the ring system. His evidence was , XUJH WKDW WZR DVSHFWV RI %DUQDUG¶V REVHUYDWLRQ DUH first-hand. He had a first-rate international reputation significant. He saw Iapetus begin to fade before it as a skillful and conservative observer and discoverer. reached the C-D ring boundary. He also saw Iapetus The only obvious fault was lack of independent con- become fainter than Enceladus too soon. Apparently, a firmation. Presumably, with four out of five favorable condition existed in the inner ring system on 1-2 indications, a majority of his colleagues would have November 1889 that does not, for whatever reason, been justified to believe him. However, Barnard, who exist now. preferred to avoid critics, offered no opportunity for Figure 13: On 30 April 1890 Paul H. Stroobant (1890) drew Saturn and its ring system. The opening angle on the rings was about - 11°, only slightly more open than what Barnard saw about six months earlier. Stroobant described the western ansa of the C ring as displaying two dark notches. The notch at the middle of the ansa had " … une forme dont il était difficile de saisir les contours exacts" [ … a shape that made it difficult to grasp the exact contours]. What is the significance of dark markings in the C ring that were seen by Terby, Elger, Stroobant and others? If they were real, were they spokes? If spokes were present in the inner ring system during the eclipse of Iapetus in 1889, would they have affected Iapetus' brightness in eclipse as Barnard recorded it in his light curve? The irregular shape that Stroobant drew for the planet's shadow on the rings is incidentally relevant. The shadow is naturally curved, but observers sometimes report non-curved shapes. In the nineteenth century some thought these anom-alous shapes were produced by topography on the rings, but for most of the twentieth century non-curvature was dismissed as an illusion. Modern critics have suggested that awareness of non-curvature may indicate an observer's susceptibility to illusion. However, Mark Bailey, David Stewart and Mark Stronge (2005) now explain non-curvature of Saturn's shadow as an optical phenomenon like the black drop in transits of Venus (after Stroobant, 1890: insert between Pp. 774 and 775, figure entitled "30 Avril 1890" with permission from the Council of l'Academie Royale des Sciences, Lettres et Beaux-Arts de Belgique). 46 James Bryan E.E. Barnard and the 1889 Eclipse of Iapetus 13 ACKNOWLEDGEMENTS HTXDWRULDORIWKH/LFN2EVHUYDWRU\Monthly Notices of the Royal Astronomical Society I thank the following people for their help. Stephen %ODLU*1DQG2ZHQ)1 7KH8%9RUELWDOSKDVH 2¶0HDUDGHVFULEHGLQGHWDLOKLVYLVXDOREVHUYDWLRQVRI FXUYHVRI5KHD'LRQHDQG7HWK\VIcarus VSRNHV$PDQGD%RVK-RVKXD&ROZHOO-HIIUH\&X]]L %RVK $6 DQG 2ONLQ &% /RZ RSWLFDO GHSWK 5LFKDUG )UHQFK 0LKDO\ +RUDQ\L 0DUN 6KRZDOWHU IHDWXUHV LQ 6DWXUQ¶V ULQJV WKH RFFXOWDWLRQ RI *6& /DXUHQFH7UDIWRQDQG$QQH9HUELVFHUDQVZHUHGVFLHQ E\ 6DWXUQ DQG LWV ULQJV Bulletin of the American WLILFTXHVWLRQV-RKQ,VOHVDGYLVHGRQYLVXDOHVWLPDWLRQ Astronomical Society RIVWHOODUPDJQLWXGHV:LOOLDP*UD\FRPSXWHGYDULRXV %UXVK6*(YHULWW&:)DQG*DUEHU( Maxwell FLUFXPVWDQFHVIRUWKHHFOLSVHRI,DSHWXV)UDQFLV(YHU on Saturn’s Rings&DPEULGJH0,73UHVV LWW DQVZHUHG TXHVWLRQV DERXW -DPHV &OHUN 0D[ZHOO %XUQKDP 6: 7KH WUDSH]LXP RI 2ULRQ Monthly )RUFRSLHVRIGRFXPHQWVDQGRWKHULQIRUPDWLRQE\DQG Notices of the Royal Astronomical Society &KDPEHUV *) A Handbook of Descriptive and DERXW %DUQDUG &OHUN 0D[ZHOO .HHOHU 0DUWK DQG Practical Astronomy. Volume 1. Fourth Edition 2[IRUG 0DXQGHU , WKDQN 'RURWK\ 6FKDXPEHUJ DQG &KHU\O &ODUHQGRQ 'DQGULGJH DW WKH 0DU\ /HD 6KDQH $UFKLYHV IRU /LFN &KDQGOHU6&-U7KHFKDQJHLQWKHJUHDWQHEXODLQ 2EVHUYDWRU\ 7HUHVD *UD\ DW 6SHFLDO &ROOHFWLRQV DQG $QGURPHGDScience 8QLYHUVLW\ $UFKLYHV IRU 9DQGHUELOW 8QLYHUVLW\ 3HWHU &OHUN0D[ZHOO-/HWWHUWR:LOOLDP7KRPVRQGDWHG +LQJOH\DQG0DU\&KLEQDOODWWKHOLEUDU\RIWKH5R\DO $XJXVW*ODVJRZ8QLYHUVLW\/LEUDU\.HOYLQ3DSHUV0 $VWURQRPLFDO6RFLHW\ >'U@6WHSKHQ:7D\ORU%$ &RRN$),,DQG)UDQNOLQ)$ 2SWLFDOSURSHUWLHVRI 3K' D KLVWRULFDO UHVHDUFKHU DQG JHQHDORJLVW LQ WKH 6DWXUQ¶VULQJV ,7UDQVPLVVLRQSmithsonian Contributions 8QLWHG.LQJGRP -DQH:HVVDWWKH1DWLRQDO0XVHXP to Astrophysics &R[ $1 HG Allen’s Astrophysical Quantities. RI 6FLHQFH DQG ,QGXVWU\ DQG -RDQQD %DOO DW 7ULQLW\ Fourth Edition1HZ 47 James Bryan E.E. Barnard and the 1889 Eclipse of Iapetus Marth, A., 1889a. Letter to W.H. Wesley, dated 14 February. Porco, C.C., and Danielson, G.E., 1982. The periodic var- Royal Astronomical Society, RAS Letters 1889. LDWLRQ RI VSRNHV LQ 6DWXUQ¶V ULQJV Astronomical Journal, Marth, A., 1889b. On the eclipse of Iapetus by Saturn and its 87, 826-833. ring-system, on November 1-2, 1889. Monthly Notices of Porco, C.C., et al., 2005. Cassini imaging science: initial the Royal Astronomical Society, 49, 427-429. UHVXOWVRQ6DWXUQ¶VULQJV and small satellites. Science, 307, Marth, A., 1889c. Letter to W. H. Wesley, dated 12 Septem- 1226-1236. ber. Royal Astronomical Society, RAS Letters 1889. Robinson, L.J., 1980. Closing in on Saturn. Sky & Telescope, Marth, A., 1889d. Letter to RAS Library Committee, dated 6 60, 481. November. Royal Astronomical Society, RAS Letters Sandel, B. R., et al., 1982. Extreme ultraviolet observations 1889. from the Voyager 2 encounter with Saturn. Science, 215, Marth, A., 1890. Letter to E.S. Holden, dated 6 February. 548-553. Mary Lea Shane Archives of the Lick Observatory, Shapin, S., 1994. A Social History of Truth. Chicago, Uni- University of California, Santa Cruz. versity of Chicago Press. Maunder, E.W., 1894. Letter to E.E. Barnard dated 2 January. Vanderbilt University Special Collections, E.E. Sheehan, W., 1988. Planets & Perception: Telescopic Views Barnard papers. and Interpretations, 1609-1909. Tucson, University of McGhee, C.A., French, R.G., Dones, L., Cuzzi, J.N., Salo, Arizona Press. H.J., and Danos, R., 2005. HST observations of spokes in Sheehan, W., 1995. The Immortal Fire Within: The Life and 6DWXUQ¶V%ULQJIcarus, 173, 508-521. Work of Edward Emerson Barnard. Cambridge, Cam- Millis, R.L., 1973. UBV photometry of Iapetus. Icarus, 18, bridge University Press. 247-252. Showalter, M.R., 1996. Saturn¶V ' ULQJ LQ WKH 9R\DJHU Millis, R.L., 1977. UBV photometry of Iapetus: results from images. Icarus, 124, 677-689. five apparitions. Icarus, 31, 81-88. Showalter, M.R., 1998. Detection of centimeter-sized Mitchell, C.J., and Horanyi, M., 2005. Where did the spokes PHWHRURLGLPSDFWHYHQWVLQ6DWXUQ¶V)ULQJScience, 282, go? American Geophysical Union Spring Meeting 2005, 1099-1102. abstract P21C-07. Smith, B.A. 1984. Future observations of planetary rings Mitchell, C.J., Horanyi, M., Havnes, O., and Porco, C.C., from groundbased observatories and earth-orbiting satel- 6DWXUQ¶VVSRNHV ORVWDQGIRXQGScience, 311, 1587- lites. In Greenberg, R., and Brahic, A. (eds.). Planetary 1589. Rings. Tucson, University of Arizona Press. Pp. 709-710. NASA/JPL/Space Science Institute, 2006a. Double-Banded Smith, B.A., et al., 1982. A new look at the Saturn system: E Ring (PIA 07803). the Voyager 2 images. Science, 215, 504-537. http://ciclops.org/view.php?id=2000 (accessed on 5 July Spahn, F., et al., 2006. Cassini dust measurements at 2006). Enceladus and implications for the origin of the E ring. NASA/JPL/Space Science Institute, 2006b. Shadow Strands Science, 311, 1416-1418. (PIA 08191). Stroobant, P., 1890. Observations de Saturne faites en 1890, http://ciclops.org/view.php?id=1995 (accessed on 2 June j O¶REVHUYDWRLUH UR\DO GH %UX[HOOHV Bulletin de L’Acad- 2006). émie Royale des Sciences, des Lettres et des Beaux-Arts de NASA/JPL/Space Science Institute, 2005a. D Ring Reve- Belgique, 3rd series, 19, 6, 772-775. lations (PIA 07714, 07715). Struve, O.W., 1853. Les dimensions des anneaux de Saturne. http://ciclops.lpl.arizona.edu/view.php?id=955 (accessed Memoires de l’academie Imperiale des Sciences de St. on 18 May 2005). Petersbourg. 6 ser.: Sciences mathematiques, physiques et NASA/JPL/Space Science Institute, 2005b. Fantasy Made naturelles. 1re ptie: Sciences mathematiques et physiques, Real (PIA 07545). T.5. (Memoires de l'academie imperiale des sciences de St. http://ciclops.org/view.php?id=1172 (accessed on 26 Petersbourg. 6 ser.: Sciences mathematiques, physiques et August 2005). naturelles; T.7), 439-475. Obituary: Albert Marth. 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Pickering, E.C., 1882. A Plan for Securing Observations of James Bryan is a Research Fellow at McDonald the Variable Stars. Cambridge (Mass.), Harvard Univ. Observatory, a branch of the University of Texas at Press. Austin. His research interests include history of Pickering, E.C., et al., 1881. Report of the committee on astronomy in early Texas, historical observations of standards of stellar magnitude. Observatory, 4, 327-329. the supernova S Andromedae in 1885, and histor- Porco, C.C., 1988. Dual periodicity in the appearance of ical visual observations of Saturn's ring system. He spokes. Bulletin of the American Astronomical Society, 20, is a member of the American Astronomical Society 852. and is a Fellow of the Royal Astronomical Society. 48